An experimental and computational study of the reaction between pent-3-en-2-yl radicals and oxygen molecules: switching from pure stabilisation to pure decomposition with increasing temperature.

We have used laser-photolysis-photoionization mass spectrometry, quantum chemical calculations, and master equation simulations to investigate the kinetics of the reaction between (E/Z)-pent-3-en-2-yl a resonance-stabilised hydrocarbon radical, and molecular oxygen. The time-resolved experiments were performed over a wide temperature range (240-750 K) at relatively low pressures (0.4-7 Torr) under pseudo-first-order conditions (excess [O2]). Helium bath gas was used in most experiments, but nitrogen was employed in a few measurements to investigate the effect of a heavier collider on the kinetics of the studied reaction. The experimental traces were directly used to optimise parameters in the master equation model using the recently implemented trace fitting feature in the MESMER program. At low temperatures (T < 300 K), the reaction proceeds by barrierless recombination reactions to form peroxyl adducts, and the radical traces are single-exponential. Between 326 K and 376 K, equilibration between the reactants and the peroxyl adducts is observed, and the radical traces are multi-exponential. Interestingly, at temperatures above 500 K, single-exponential decays were again observed, although the reaction is much slower than at low temperatures. The master equation simulations revealed that at both low and high temperatures, the radical decay rate is governed by a single eigenvalue. At low temperatures, this eigenvalue corresponds to recombination reactions, and at high temperatures to the phenomenological formation of bimolecular products. Between low and high temperatures (the exact temperature thresholds depend on [O2]), there is a region of avoided crossing in which the rate coefficient "jumps" from one eigencurve to the other. Although chemically significant eigenvalues are not well separated from internal energy relaxation eigenvalues at elevated temperatures (600 K at 0.01 bar, 850 K at 100 bar), we observed that many of the Bartis-Widom rate coefficients produced by the master equation model were valid up to 1500 K. Our simulations predict that the most important reaction channel at high temperatures is the formation of (E/Z)-penta-1,3-diene and hydroperoxyl. The experimentally constrained master equation model was used to simulate the title reaction over a wide range of conditions. To facilitate the use of our results in autoignition and combustion models, modified Arrhenius representations are given for the most important reaction channels.

Imperfect mixing as a dominant factor leading to stochastic behavior: a new system exhibiting crazy clock behavior.

It is clearly demonstrated that the arsenous acid-periodate reaction displays crazy-clock behavior when a statistically meaningful number of kinetic runs are performed under "exactly the same" conditions. Both extensive experimental and numerical simulation results gave convincing evidence that the stochastic feature of the title reaction originates from the imperfection of the mixing process, and neither local random fluctuation nor initial inhomogeneity alone is capable of explaining adequately the observed phenomena. Imperfect mixing is manifested-in practice-in the unintentional and inherent formation of dead volumes where the concentration of the reactants may even significantly differ from the ones measured in the case of a completely uniform concentration distribution, and the system may spend enough time there under imperfectly mixed conditions to complete the nonlinear chemical process. Furthermore, it is also shown that a more efficient mixing, i.e. a smaller dead volume size and shorter residence time being spent in the dead volume, does not necessarily mean Landolt times are smaller than the one measured under completely homogeneous conditions. Evidently, the "initial" concentration of the reagents in the dead volume-and of course in the rest of the solution-greatly influences the Landolt time to be measured in the case of an individual kinetic run and may therefore show either positive or negative deviation from the Landolt time for the completely homogeneous state. As a result, less efficient mixing may either accelerate or decelerate the rate of a nonlinear autocatalytic reaction at a macroscopic volume level.

Clarifying the Equilibrium Speciation of Periodate Ions in Aqueous Medium.

Equilibria of periodate ion were reinvestigated in aqueous solution by using potentiometric titration, UV and Raman spectroscopies, and gravimetry simultaneously at 0.5 M ionic strength and at 25.0 ± 0.2 °C. Stepwise acid dissociation constants of orthoperiodic acid were found to be pK1 = 0.98 ± 0.18, pK2 = 7.42 ± 0.03, and pK3 = 10.99 ± 0.02, as well as pK2 = 7.55 ± 0.04 and pK3 = 11.25 ± 0.03 in the presence of sodium nitrate and sodium perchlorate as background salts, respectively. pK1 cannot be determined unambiguously from our experiments in the presence of sodium perchlorate. The molar absorptivity spectrum of H4IO6- and H3IO62- was determined in the range of 215-335 nm, as major species of periodate present from slightly acidic to slightly alkaline conditions. The solubility of periodate decreases significantly under alkaline conditions, and it was determined to be (2.8 ± 0.4) mM by gravimetry, under our experimental conditions. None of these studies gave any clear evidence for an ortho-meta equilibrium and the frequently invoked dimerization of periodate. All measurements can quantitatively be described by the presence of orthoperiodic acid and its three successive deprotonation steps.

Kinetics and Mechanism of the Oxidation of Thiourea Dioxide by Iodine in a Slightly Acidic Medium.

The thiourea dioxide (TDO)-iodine reaction was investigated spectrophotometrically monitoring the consumption of total amount of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered slightly acidic medium. The nitrogen- and carbon-containing products were found to be ammonium ion and dissolved carbon dioxide, respectively, while from sulfur part sulfate ion was exclusively detected, when fresh TDO solution was used. The stoichiometry of the reaction was established as 2I2 + TDO + 4H2O → SO42- + 2NH4+ + 4I- + CO2 + 4H+ indicating a strict 2:1 stoichiometric ratio. However, using aged TDO solution this stoichiometric ratio is shifted to lower values suggesting the formation of elementary sulfur augmented by the 2TDO + I2 + 4H2O → S + SO42- + 4NH4+ + 2I- + 2CO2 hypothetical limiting stoichiometry. We also confirmed experimentally that in aqueous solution TDO slowly rearranges into an unindentified species. This species then produces elementary sulfur at a later stage of the aging process via subsequent reactions accounting for a loss of reducing power. The direct reaction between TDO and iodine was found to be relatively rapid and completed within seconds in absence of initially added iodide ion. Formation of the latter ion, however, strongly inhibits the oxidation process; hence, the system is autoinhibitory with respect to iodide ion. Furthermore, increase of pH markedly accelerates the reaction as well. These observations suggest that a short-lived steady-state intermediate (iodinated TDO) is produced in a rapid pre-equilibrium, where iodide and hydrogen ions are also involved. A nine-step kinetic model, to be able to describe the most important characteristics of the experimental curves with four fitted parameters, is proposed and discussed.

Reactions of aquacobalamin and cob(II)alamin with chlorite and chlorine dioxide.

Reactions of aquacobalamin (H2O-Cbl(III)) and its one-electron reduced form (cob(II)alamin, Cbl(II)) with chlorite (ClO2-) and chlorine dioxide (ClO 2• ) were studied by conventional and stopped-flow UV-Vis spectroscopies and matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS). ClO2- does not react with H2O-Cbl(III), but oxidizes Cbl(II) to H2O-Cbl(III) as a major product and corrin-modified species as minor products. The proposed mechanism of chlorite reduction involves formation of OCl- that modifies the corrin ring during the course of reaction with Cbl(II). H2O-Cbl(III) undergoes relatively slow destruction by ClO 2• via transient formation of oxygenated species, whereas reaction between Cbl(II) and ClO 2• proceeds extremely rapidly and leads to the oxidation of the Co(II)-center.

Kinetics and Mechanism of the Chlorite-Periodate System: Formation of a Short-Lived Key Intermediate OClOIO3 and Its Subsequent Reactions.

The chlorite-periodate reaction has been studied spectrophotometrically in acidic medium at 25.0 ± 0.1 °C, monitoring the absorbance at 400 nm in acetate/acetic acid buffer at constant ionic strength (I = 0.5 M). We have shown that periodate was exclusively reduced to iodate, but chlorite ion was oxidized to chlorate and chlorine dioxide via branching pathways. The stoichiometry of the reaction can be described as a linear combination of two limiting stoichiometries under our experimental conditions. Detailed initial rate studies have clearly revealed that the formal kinetic orders of hydrogen ion, chlorite ion, and periodate ion are all strictly one, establishing an empirical rate law to be d[ClO2]/dt = kobs[ClO2(-)][IO4(-)][H(+)], where the apparent rate coefficient (kobs) was found to be 70 ± 13 M(-2) s(-1). On the basis of the experiments, a simple four-step kinetic model with three fitted kinetic parameters is proposed by nonlinear parameter estimation. The reaction was found to proceed via a parallel oxygen transfer reaction leading to the exclusive formation of chlorate and iodate as well as via the formation of a short-lived key intermediate OClOIO3 followed by its further transformations by a sequence of branching pathways.

Compatible Mechanism for a Simultaneous Description of the Roebuck, Dushman, and Iodate-Arsenous Acid Reactions in an Acidic Medium.

The iodine-arsenous acid (Roebuck), iodide-iodate (Dushman), and iodate-arsenous acid reactions have been studied simultaneously by a stopped-flow technique by monitoring the absorbance-time profiles at the isosbestic point of the I2/I3(-) system (468 nm). Using the well-accepted rate coefficients of iodine hydrolysis, we have proven that iodine is the kinetically active species of the iodine-arsenous acid reaction. Strong iodide inhibition of this system is explained by a rapidly established equilibrium between iodine and arsenous acid to produce an iodide ion, a hydrogen ion, and a short-lived intermediate H2AsO3I, which is shifted far to the left. Taking into consideration the generally accepted kinetic model of the Dushman reaction where I2O2 plays a key role to account for all of the most important observations in this subsystem and a sequence of simple formal oxygen-transfer reactions between arsenous acid and iodic acid as well as iodous acid and hypoiodous acid, we propose a 13-step comprehensive kinetic model, including seven rapidly established equilibria with only six fitted parameters, that is able to explain all of the most important characteristics of the kinetic curves of all of the title systems both individually and simultaneously.

A Simple Kinetic Model for Description of the Iodate-Arsenous Acid Reaction: Experimental Evidence of the Direct Reaction.

The autocatalytic iodate-arsenous acid reaction was investigated by a stopped-flow instrument under strongly acidic medium (pH ≤ 1) by monitoring the absorbance-time profiles at 468 nm. The kinetic traces were found to exhibit a perfect sigmoidal shape in stoichiometric excess of iodate with a well-defined and reproducible induction period that depends on the initial concentration of the reactants as well as on the pH. All the experimental curves can be globally fitted by a simple kinetic model involving the direct reaction between the reactants to produce iodide ion, the Dushman and the Roebuck reactions, and two rapid equilibria. Our measurements along with simultaneous evaluation of the kinetic traces clearly support that indeed the initiation reaction exists at strongly acidic conditions and contributes to the overall kinetics. The measured traces cannot be described adequately by the iodide ion impurity-driven Dushman and Roebuck reactions with assuming no direct reaction at all.

Initial inhomogeneity-induced crazy-clock behavior in the iodate-arsenous acid reaction in a buffered medium under stirred batch conditions.

It is unambiguously demonstrated that in the case of an autocatalytic reaction, initial inhomogeneities induced by the imperfectly mixed part of the overall volume may result in a serious irreproducibility of the individual kinetic runs. A statistically meaningful number of repetitions, however, gives rise to a reproducible cumulative probability distribution curve often referred to as a support of the stochastic feature. The iodate-arsenous acid reaction being autocatalytic with respect to both iodide and hydrogen ions displays clock behavior. However, the time lag necessary for the appearance of iodine, even in buffered solution, varies in an apparently random manner. Careful analysis of the variation of the different parameters like stirring rate, overall volume, geometry of the reactor and the way of mixing the reactants led us to conclude that the fate of the individual samples is determined at the initial stage when the reacting system is per se inhomogeneous. The place, the size of the so-called ignition volume, where the reacting system is imperfectly stirred, as well as the residence time spent there by the imperfectly mixed reactants all seem to depend on external factors.